Someone attempted to sabotage a Horace Mann senior’s college admissions chances by sending a letter with untrue and damaging information to schools the student is applying to, according to a letter sent to the senior class this week.

“In 20 years of college admissions I have never witnessed anything so disrespectful,” wrote Canh Oxelson, director of college counseling at the Bronx private school. “For a student to have worked so hard for so many years, only to see those efforts jeopardized by an act of sabotage is absolutely unconscionable.”

The act undermined the entire senior class by raising questions about the integrity of the school in the minds of admissions officers and feeding “the perception that we are hypercompetitive to the point of being unhealthy,” Mr. Oxelson said.

Officials at Horace Mann declined to comment.

News of the anonymous letter surfaced several weeks ago, Mr. Oxelson said, noting that he has been working with admissions officers to “minimize the damage.”

College admissions are central to the identity of the city’s top private schools, which derive much of their prestige from their students’ acceptance letters.

“When you have so much competition, you want something so much and you’ve been working so hard for admission—whether it’s kindergarten or college—it can truly make the most sane person lose it,” said Suzanne Rheault, the CEO of Aristotle Circle, a tutoring and admissions consulting company.

Ms. Rheault said she had heard from several parents at Horace Mann who were “very upset.”

She praised the school for being upfront about the issue. “Transparency definitely helps,” she said.

“Everybody was surprised and the children were particularly surprised because they feel close and a strong sense of camaraderie,” said Mitchell Kline, a parent at the school. But Mr. Kline added, “we have full faith that the school handled this well.”

Mr. Oxelson wrote in his letter that students and counselors at Horace Mann had spent weeks strategizing about how to provide admissions officers with the most positive impression of the school.

He said that included “doing the little things” such as greeting visitors politely, acting courteous on campus visits and demonstrating “kindness and respect” to “help outsiders understand who we really are.”

But the school’s reputation becomes “empty” when “it’s clear we aren’t living up to our core values,” Mr. Oxelson said.